Previous builds failed due to a missing config.ini.php.
Using the before_script command we now copy a configuration file
to config/. We also create a database for tests.
PHPUnit complained about missing XDebug - so this is added.

I'm pretty sure I had this problem before. Usually, it completely crashes either the computer (swapping) or php/phpunit. One problem comes when generating & displaying the backtrace for the test failures. Some backtrace are just too huge to be displayed, and result in crash or minutes of waiting. I think this is the problem here. I think that, once all tests pass again, this can be switched back to "full build" to try again.